And some of the first photos it snapped and transmitted to Earth told a mysterious tale of this icy world that still has scientists scratching their heads. Mountains of water ice and nitrogen snow are just a couple of the perplexing possibilities on Pluto.

Now, New Horizons has sent back some of the most detailed photos yet of Pluto's surface that have doubled the amount that we can see in super-fine detail.

"Pluto is showing us a diversity of landforms and complexity of processes that rival anything we've seen in the solar system," said New Horizons Principal Investigator Alan Stern in a NASA statement. "If an artist had painted this Pluto before our flyby, I probably would have called it over the top — but that's what is actually there."

These latest shots reveal the most heavily cratered portion of Pluto ever seen. Because other features on Pluto are surprisingly smooth — which suggests a younger surface smoothed over time by geological activity — this heavily cratered region is the oldest region of Pluto observed so far, NASA said.

The oldest region borders one of the youngest on Pluto, informally named Sputnik Planum, which is the left lobe of the heart-shaped feature. Here's where on Pluto we're looking:

JPL/NASAScientists suspect the smooth, light color in Sputnik Planum could be from nitrogen snow. Here's a close-up of Sputnik Planum, with the heavily cratered section shown in black toward the bottom. This photo covers about 1,000 miles left to right:

"Seeing dunes on Pluto — if that is what they are — would be completely wild, because Pluto's atmosphere today is so thin," said William B. McKinnon, in the NASA statement. "Either Pluto had a thicker atmosphere in the past, or some process we haven't figured out is at work. It's a head-scratcher."

What caused this region to remain so unchanged over billions of years while its neighbor, Sputnik Planum, grew a mysteriously smooth, young surface is still under debate.